1. Field of the Invention
This invention relates to magnetic recording disk drives that have a fly-height actuator for controlling the spacing between the read/write head and the disk, and more particularly to a system and method for determining head-disk contact in such disk drives, including disk drives with bit-patterned media (BPM).
2. Description of the Related Art
Magnetic recording hard disk drives use a read/write transducer or head mounted on a head carrier for reading and/or writing data to the disk. The head carrier is typically an air-bearing slider attached to an actuator arm by a suspension and positioned very close to the disk surface by the suspension. There is typically a stack of disks in the disk drive with a slider-suspension assembly associated with each disk surface in the stack.
The separation or spacing between the head and the disk surface is called the fly height. The slider has a disk-facing air-bearing surface (ABS) that causes the slider to ride on a cushion or bearing of air generated by rotation of the disk. The slider is attached to a flexure on the suspension and the suspension includes a load beam that applies a load force to the slider to counteract the air-bearing force while permitting the slider to “pitch” and “roll”. The flying dynamics of the slider and thus the fly height are influenced by factors such as the rotation speed of the disk, the aerodynamic shape of the slider's ABS, the load force applied to the slider by the suspension, and the pitch and roll torques applied to the slider by the suspension.
Disk drives have been proposed that use a fly-height actuator for changing the spacing between the head and the disk surface. One type of fly-height actuator is a thermal actuator with an electrically-resistive heater located on the slider near the head. When current is applied to the heater the heater expands and causes the head to “protrude” and thus move closer to the disk surface. Other fly-height actuators for moving the head relative to the slider include electrostatic microactuators and piezoelectric actuators. Another type of fly-height actuator, also based on thermal, electrostatic or piezoelectric techniques, changes the head-disk spacing by altering the air-flow or the shape of the slider's ABS.
The fly-height actuator must be accurately calibrated so that the head-disk spacing can be controlled. The calibration requires that the value of the control signal that results in onset of head-disk contact (HDC) be known. If the fly-height actuator is a thermal actuator with an electrically-resistive heater located on the slider near the head, this control signal value is the value of heater power that results in HDC.